The latest calls for the environmental protection and the reduction of the life cycle costs of commodities have had repercussions on fuel related members, such as a fuel tank, a fuel pipe and the like, which are components of an automobile.
A tank produced by using a lead-plated steel sheet as the base material and applying paint coating thereto has long been used practically as the fuel tank of an automobile, but, in view of the latest requirements for the environmental protection, new base materials free of lead are being developed. In addition, taking a serious view of the environmental impact of painting work itself, the needs for the elimination of the painting which has been deemed indispensable for the conventional plated steel sheets have arisen.
As a background to the above movements, there is a situation in which the LEV-II regulation is to be prescribed in California, U.S.A. in 2002 and carmakers will be obliged to guarantee the service life of a fuel tank for 15 years of use or through 150,000 miles of running.
Research and development activities are under way at present to develop a fuel tank satisfying the regulation, focusing on the following three alternative base materials: a plated steel material, on the premise that painting is applied as stated above, a resin and a stainless steel on the premise that painting is not applied. Among the three base materials, the resin has a problem in recyclability, and the plated steel material requires heavy painting and therefore is liable to raise the costs and to reproduce the environmental problems.
Consequently, what is attracting attention at present is a fuel tank produced by using a stainless steel type base material which has no problem with respect to the corrosion resistance of the inner surface. Attempts are being made along this line of thought to minimize the cost/performance by eliminating the painting work which has been deemed indispensable for securing the corrosion resistance of the outer surface of a conventional plated steel material while the good recyclability of a ferrous material is maintained.
The portion liable to a problem regarding the corrosion resistance of an outer surface in the environment of deicing salt is a welded portion inevitably involved in the fabrication of a fuel tank. Even though non-welded portions have good corrosion resistance, a welded portion has the danger of generating local corrosion such as pitting corrosion, crevice corrosion and stress corrosion cracking (SCC) because the corrosion resistance of a stainless steel base material is significantly damaged by the destruction of a passivated film formed on the surface of the base material or the formation of gapped portions which are caused by the welding.
Multipurpose austenitic stainless steel such as SUS304L is considered a candidate material for solving the above problems, but a shortcoming with the material is that SCC is generated under a salt damage condition. In view of the above problem, ferritic stainless steel is useful as a material for avoiding the SCC problem. However, the material containing small amounts of alloying elements such as Cr and Mo does not always exhibit sufficient corrosion resistance in a salt damage environment.
The extended service life as described above is required not only of a fuel tank but also of a fuel pipe. A fuel pipe undergoes bending and expansion work, then brazing or welding work and assembly with metal fittings, and, as a result, gapped portions are formed. Thus, a fuel pipe has the problem of local corrosion on the outer surface just like a fuel tank.
As explained above, when stainless steel is used for the fuel related members of an automobile, although the corrosion problem of the inner surface exposed to a fuel environment is avoided, the resistance against the local corrosion such as pitting corrosion, crevice corrosion and SCC of the outer surface exposed to deicing salt constitutes a problem requiring solution.